Remove dead code from Convert module

This pull request is *dead* simple (ha), it removes unused functions from the `convert` module as get closer to removing it all together.
Two major roadblocks until that can happen are:

* VarInt, and
* hexToBytes/bytesToHex usage in tests and Transaction/Script

Both require complete conversion of their dependent modules before we can refactor them out.
However VarInt will still need to exist in some form, so its up for debate how that should be implemented (it is only used in very specific places, we can probably follow bitcoin-qt's lead here on how they simply have a free function for it).